Articles | Volume 17, issue 22
Atmos. Chem. Phys., 17, 13819–13831, 2017
Atmos. Chem. Phys., 17, 13819–13831, 2017

Research article 20 Nov 2017

Research article | 20 Nov 2017

The role of highly oxygenated molecules (HOMs) in determining the composition of ambient ions in the boreal forest

Federico Bianchi1,*, Olga Garmash1, Xucheng He1, Chao Yan1, Siddharth Iyer2, Ida Rosendahl3, Zhengning Xu4, Matti P. Rissanen1, Matthieu Riva1, Risto Taipale1, Nina Sarnela1, Tuukka Petäjä1, Douglas R. Worsnop1,5, Markku Kulmala1, Mikael Ehn1, and Heikki Junninen1,6 Federico Bianchi et al.
  • 1Department of Physics, University of Helsinki, 00014 Helsinki, Finland
  • 2Department of Chemistry, University of Helsinki, 00014 Helsinki, Finland
  • 3Arctic Research Centre, Aarhus University, 8000 Aarhus, Denmark
  • 4Joint International Research Laboratory of Atmospheric and Earth System Sciences, School of Atmospheric Sciences, Nanjing University, 210023 Nanjing, China
  • 5Aerodyne Research, Inc., Billerica, MA 01821, USA
  • 6Institute of Physics, University of Tartu, 50090 Tartu, Estonia
  • *Invited contribution by Federico Bianchi, recipient of the EGU Arne Richter Award for Outstanding Early Career Scientists 2017.

Abstract. In order to investigate the negative ions in the boreal forest we have performed measurements to chemically characterise the composition of negatively charged clusters containing highly oxygenated molecules (HOMs). Additionally, we compared this information with the chemical composition of the neutral gas-phase molecules detected in the ambient atmosphere during the same period. The chemical composition of the ions was retrieved using an atmospheric pressure interface time-of-flight mass spectrometer (APi-TOF-MS) while the gas-phase neutral molecules (mainly sulfuric acid and HOMs) were characterised using the same mass spectrometer coupled to a nitrate-based chemical ionisation unit (CI-APi-TOF). Overall, we divided the identified HOMs in two classes: HOMs containing only carbon, hydrogen and oxygen and nitrogen-containing HOMs or organonitrates (ONs). During the day, among the ions, in addition to the well-known pure sulfuric acid clusters, we found a large number of HOMs clustered with nitrate (NO3) or bisulfate (HSO4), with the first one being more abundant. During the night, the distribution of ions, mainly composed of HOM clustered with NO3, was very similar to the neutral compounds that are detected in the CI-APi-TOF as adducts with the artificially introduced primary ion (NO3).

For the first time, we identified several clusters containing up to 40 carbon atoms. These ions are formed by up to four oxidised α-pinene units clustered with NO3. While we know that dimers (16–20 carbon atoms) are probably formed by a covalent bond between two α-pinene oxidised units, it is still unclear what bonding formed larger clusters.

Finally, diurnal profiles of the negative ions were consistent with the neutral compounds revealing that ONs peak during the day while HOMs are more abundant at night-time. However, during the day, a large fraction of the negative charge is taken up by the pure sulfuric acid clusters causing differences between ambient ions and neutral compounds (i.e. less available charge for HOM and ON).

Short summary
Naturally charged highly oxidised molecules (HOMs) were characterized using advanced mass spectrometers. Two different classes of compounds, clustered with the nitrate and bisulfate ions, were identified: HOMs containing only carbon, hydrogen and oxygen and nitrogen-containing HOMs or organonitrates (ONs). They exhibit strong diurnal variations where HOMs peak during night and ONs during day. Finally, large clusters containing up to 40 carbon atoms (four oxidized α-pinene units) were observed.
Final-revised paper